1. Field of the Invention
This invention relates to a device for automatically feeding stacked sheets one by one to a prescribed position in an image processing device such as a copying machine and image scanner, and more particularly to an automatic sheet feeding device capable of smoothly and stably transferring sheets one by one and materializing miniaturization of system without decreasing the sheet transferring efficiency thereof.
2. Description of the Prior Art
In an electrophotographic copying machine, for example, sheet feeding devices for automatically transferring stacked original sheets (manuscript sheets) to be copied one by one to an original reading position have come to be adopted in general. There is a typical sheet feeding device of this sort which, as illustrated in FIG. 1(A), comprises a sheet stacking tray 2 disposed over the original reading position Pr defined on a transparent platen 1, and a non-linear sheet guide passage 3 along which the document original sheet s to be copied is transferred from the tray 2 to the original reading position Pr. The purpose for which the sheet stacking tray 2 is disposed over the original reding position Pr is to lessen a space for the sheet feeding device to occupy; namely, to improve the space factor of the device. However, in such a structure in which the sheet stacking tray 2 is over the original reading position Pr, it is necessary to curve the sheet guide passage 3 extending from the tray 2 to the original reading position Pr. In order to further improve the space factor of the device, the sheet guide passage must be shortened and reduced in its effective curvature. The reduction of the curvature of the sheet guide passage, nevertheless, exerts adverse effects on the efficiency of transferring the sheet.
As illustrated in FIG. 1(B), the conventional sheet feeding device for transferring a stack of sheets Os one by one from the aforesaid tray 2 to the original reading position Pr basically comprises at least one pickup roller 4 for drawing out one document original sheet s from the stacked sheets Os on the tray 2, a sheet separation means 5 consisting of at least one set of sheet separation roller 5a and friction pad 5b, and a sheet skew correction means 6 for correcting the sheet obliquely advancing along the passage 3, which consists of at least one pair of skew correction rollers 6a, 6b and a sheet sensor 6c. These elements are arranged along the sheet guide passage 3 defined between an outer guide plate 3a and an inner guide plate 3b between the tray 2 and the original reading position Pr.
In transferring the original sheet s to the original reading position Pr by use of the sheet feeding device as noted above, the pickup roller 4 and sheet separation roller 5a are rotated in the sheet feeding direction to thereby draw out and send one original sheet s from the stacked sheets Os into the sheet guide passage 3. When the sheet s advancing along the sheet guide passage 3 reaches the sheet sensor 6c, a timer 7 is operated to effect a time delay function so that the sheet separation roller 5a stops immediately after the sheet s reaches the paired skew correction rollers 6a, 6b and thereafter rotating the skew correction rollers 6a, 6b in the sheet feeding direction to forward the sheet s. Thus, sheet skew correction can be fulfilled through the aforenoted time delay function.
Even if one or more superfluous sheets are drawn out from the stacked sheets Os together with the sheet s to be copied by rotating the pickup roller 4 in the sheet feeding direction, only one sheet (the lowermost sheet in the illustrated example) is permitted to pass into the sheet guide passage 3 by the action of the sheet separation means 5.
Now, the problem suffered by the conventional sheet feeding device as touched upon above is that if the curvature of the curved sheet guide passage is increased in order to make the device compact, it becomes difficult to obtain smooth flow of the sheet along the sheet guide passage and effectively fulfill the sheet skew correction function, In other words, to put the sheet skew correction into practice with certainty, the sheet advancing along the passage 3 must come to the contact point of the paired skew correction rollers 6a, 6b as straight as possible without being restricted. Hence, it is desired to form a possibly long straight path 8 by flattening the inner guide plate 3b behind the skew correction rollers 6a, 6b relative to the sheet feeding direction. As a result, there is inevitably formed a sharply curved ridgeline 9 at the entrance of the straight path 8.
Still, the sharply curved ridgeline 9 entails a problem such as difficulty in smoothly forwarding the sheet along the passage 3. To be more specific, because the skew correction rollers 6a, 6b rotate in the sheet feeding direction to draw the sheet s after the sheet separation roller 5a serving to advance the sheet s stops, a tension force is exerted into the sheet between the sheet separation means 5 and the skew correction means 6 as specified in FIG. 1(B). Consequently, since the sheet s advancing along the passage 3 comes in pressure contact with the sharply curved ridgeline 9 due to the tension force exerted into the sheet, the contact resistance of the sheet s with respect to the sharply curved ridgeline 9 becomes large with the result that the sheet s is at times prevented from advancing. In addition, the sheet s advancing along the passage chafes against the ridgeline 9 to thereby take scratches and produce unpleasant harsh noises.
Also, collision of the leading end of the sheet s being introduced into the passage 3 with the outer guide plate 3a defining the passage 3 may obstruct the advance of the sheet s and makes unpleasant harsh noises. Moreover, when the sheet s which is drawn to advance by the skew correction rollers 6a, 6b is released from the sheet separation means 5, the rear end of the sheet s having an elastic force springs back to beat against the outer guide plate 3a, consequently to produce unpleasant noises.
As will be understood from the foregoing, in order to eliminate the aforementioned various drawbacks suffered by the conventional sheet feeding device, the sheet guide passage extending from the sheet separation means 5 to the skew correction means 6 had to be so far made small in effective curvature, i.e. curved sharply. The conventional sheet feeding device could be reduced in height only to a limited extend; that is to say, the overall size thereof could not be made small.